The present application relates to an electrode that includes an active material layer on a current collector and to a secondary battery that uses the electrode. The present application further relates to a battery pack, an electric vehicle, an electric power storage system, an electric power tool, and an electronic apparatus that use the secondary battery.
In recent years, various electronic apparatuses such as mobile phones and personal digital assistants (PDAs) have been widely used, and it has been demanded to further reduce the size and the weight of the electronic apparatuses and to achieve their long life. Accordingly, a battery, in particular, a small and light-weight secondary battery capable of providing high energy density has been developed as an electric power source. In these days, it has been considered to apply such a secondary battery to various other applications that may include, for example, battery packs attachably and detachably mounted on the electronic apparatuses or the like, electric vehicles such as electric automobiles, electric power storage systems such as home electric power servers, and electric power tools such as electric drills.
As secondary batteries, secondary batteries that obtain a capacity by utilizing various charge and discharge principles have been proposed. Specifically, secondary batteries that utilize insertion and extraction of electrode reactants such as lithium ions are considered promising, since such secondary batteries provide higher energy density than batteries such as lead batteries and nickel cadmium batteries.
A secondary battery includes a cathode, an anode, and an electrolytic solution. The cathode includes a cathode active material layer on a cathode current collector. The cathode active material layer includes a cathode active material that contributes to charge and discharge reactions. As the cathode active material, for example, a lithium composite oxide such as LiCoO2, LiNiO2, and LiMn2O4, a lithium phosphate compound such as LiFePO4, or the like may be typically used.
The cathode is typically formed by the following procedures. First, a cathode active material and a material such as a cathode binder are dispersed to an organic solvent, to thereby obtain slurry. Subsequently, the slurry is coated on a cathode current collector, and the resultant is then dried to form a cathode active material layer. Lastly, the cathode active material layer is compression-molded with the use of, for example, a roll-pressing machine and/or the like.
The configuration of the cathode largely influences the performance of the secondary battery. Therefore, various studies have been made on the configuration of the cathode. For example, to obtain superior charge and discharge characteristics, a lithium metal phosphate compound (LixAyPO4, where A is an element such as Fe, and x and y satisfy 0<x<2 and 0<y≦1) with a minute particle diameter, or a compound that has a predetermined X-ray diffraction characteristics (LiMPO4, where M is an element such as Fe) are used (for example, see Japanese Patent No. 4190930 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2008-541364). To improve overcharge characteristics etc., for example, the cathode active material layer that includes a plurality of layers is formed, or the compositions of the cathode active materials in the respective layers are made different from one another (for example, see Japanese Unexamined Patent Application Publication Nos. 2007-035589, 2009-245827, and 2001-338639). To improve handling in preparing the electrode, for example, a lithium manganese composite oxide (LiaMnbO4, where a and b satisfy 1≦a≦1.2, and 1.5≦b≦2) is used as the cathode active material, and a degree of circularity of a second particle (projection image) of the cathode active material is employed (for example, see Unexamined Japanese Patent Application Publication No. 2002-274853). It is to be noted that a degree of circularity of an anode active material such as natural graphite (granulated body) is employed in addition to that of the cathode active material (for example, see Japanese Unexamined Patent Application Publication No. 2007-242282).